Style/Lambda with unused argument bug

[rramsden]

Context

When using --auto-correct with a "stabby lambda" function where arguments are unused results in syntactically incorrect code:

c = -> event do
  puts 'Hello world'
end

Is autocorrected to the following:

c = lambda(_) do |_event|
  puts 'Hello world'
end

Which returns the following error when executed:

test.rb:3:in `<main>': undefined local variable or method `_' for main:Object (NameError)

---

Expected behavior

Should not inject _ arguments into lambda e.g. lambda(_) this is not allowed in Ruby. The result should be the following code:

c = lambda do |_event|
  puts 'Hello world'
end

Actual behavior

Inserts _ as an argument to lambda function e.g. lambda(_)

Steps to reproduce the problem

1. Insert the example code in the above context into a file.
2. Run rubocop --auto-correct on the file.
3. Try executing the formatted code, it will not work.

RuboCop version

$ [bundle exec] rubocop -V
0.67.2 (using Parser 2.6.2.1, running on ruby 2.4.5 x86_64-darwin18)

Ruby version

$ ruby -v
ruby 2.4.5p335 (2018-10-18 revision 65137) [x86_64-darwin18]

[koic]

Thanks for your feedback. I opened a PR #6958.